Fig. 1: Population samples and backcross design.

a Timeline of the Bicyclus anynana Leiden/Liverpool lab population, showing the samples genotyped in this study, their relationship to each other and the order in which they were analysed (1–5). b Design of daughter–father backcross, highlighting the identify (by descent) of Z chromosome segments (denoted by ƶ_i, where i refers to the identity of the paternal Z in generation P). The position of the recombination breakpoints, indicated by change in colour, is arbitrary, as is which of the daughters to backcross (recombination does not occur in female Lepidoptera). Z_F and Z_M refer to whether the Z copy came from the father (F) or mother (M). This design is expected to generate F₂ brothers that differ with respect to which portion of the Z is either homozygous (hom) or heterozygous (het). Autosomes (not shown) are expected to experience the same degree of inbreeding, regardless of sex (probability of identity by descent ~0.25, averaged across all offspring, assuming zero coancestry between P-generation parents). To assess the fertility of F₂ males, they were mated to unrelated outbred females. The F₂ generation is referred to as BF₁ (backcross F₁).